1. Field of the Invention
The present invention relates to a method for producing a carbon nanotube aggregate, and more particularly to a method for producing a bundle type carbon nanotube aggregate with controlled bulk density.
2. Description of the Related Art
Carbon nanostructures (CNSs) refer collectively to nano-sized carbon structures having various shapes, such as nanotubes, nanohairs, fullerenes, nanocones, nanohorns, and nanorods. Carbon nanostructures can be widely utilized in a variety of technological applications because they possess excellent characteristics.
Particularly, carbon nanotubes (CNTs) are tubular materials consisting of carbon atoms arranged in a hexagonal pattern and have a diameter of approximately 1 to 100 nm. Carbon nanotubes exhibit insulating, conducting or semiconducting properties depending on their inherent chirality. Carbon nanotubes have a structure in which carbon atoms are strongly covalently bonded to each other. Due to this structure, carbon nanotubes have a tensile strength approximately 100 times that of steel, are highly flexible and elastic, and are chemically stable.
Carbon nanotubes are divided into three types: single-walled carbon nanotubes (SWCNTs) consisting of a single sheet and having a diameter of about 1 nm; double-walled carbon nanotubes (DWCNTs) consisting of two sheets and having a diameter of about 1.4 to about 3 nm; and multi-walled carbon nanotubes (MWCNTs) consisting of three or more sheets and having a diameter of about 5 to about 100 nm.
Carbon nanotubes are being investigated for their commercialization and application in various industrial fields, for example, aerospace, fuel cell, composite material, biotechnology, pharmaceutical, electrical/electronic, and semiconductor industries, due to their high chemical stability, flexibility and elasticity. However, carbon nanotubes have a limitation in directly controlling the diameter and length to industrially applicable dimensions for practical use owing to their primary structure. Accordingly, the industrial application and use of carbon nanotubes are limited despite their excellent physical properties.
Carbon nanotubes are generally produced by various techniques, such as arc discharge, laser ablation, and chemical vapor deposition. However, arc discharge and laser ablation are not appropriate for mass production of carbon nanotubes and require high arc production costs or expensive laser equipment. Chemical vapor deposition using a vapor dispersion catalyst has the problems of a very low synthesis rate and too small a size of final carbon nanotube particles. Chemical vapor deposition using a substrate-supported catalyst suffers from very low efficiency in the utilization of a reactor space, thus being inappropriate for mass production of carbon nanotubes. Thus, studies on catalysts and reaction conditions for chemical vapor deposition are currently underway to increase the yield of carbon nanotubes.
Catalytically active components of the catalysts usually take the form of oxides, partially or completely reduced products, or hydroxides. The catalysts may be, for example, supported catalysts or coprecipitated catalysts, which can be commonly used for carbon nanotube production. Supported catalysts are preferably used for the following reasons: supported catalysts have a higher inherent bulk density than coprecipitated catalysts; unlike coprecipitated catalysts, supported catalysts produce a small amount of a fine powder with a size of 10 microns or less, which reduces the possibility of occurrence of a fine powder due to attrition during fluidization; and high mechanical strength of supported catalysts effectively stabilizes the operation of reactors.
Particularly, carbon nanotubes whose bulk density is at a constant level are advantageous for the production of an aggregate of carbon nanotubes using a fluidized bed reactor.
A need also exists for bundle type carbon nanotubes that have a small diameter and are readily dispersible in and miscible with polymers during compounding with the polymers, thus being suitable for use in the manufacture of composite materials with improved physical properties.